The utility of crosslinked polymer gels as a permeability reducing agent in subterranean hydrocarbon-bearing formations to facilitate hydrocarbon recovery therefrom has long been known in the art. U.S. Pat. No. 3,762,476 to Gall is representative of conventional teaching in the art of its time. Gall discloses a conformance improvement treatment process, wherein a permeability reducing crosslinked polymer gel is formed in situ by injecting an aqueous slug containing a crosslinkable water-soluble polymer into a subterranean treatment region followed in sequence by an aqueous slug containing a crosslinking agent containing a polyvalent metal cation. Useful polyvalent metal cations listed in Gall include iron(II), iron(III), aluminum(III), chromium(III), calcium(II), and magnesium(II).
Sequential injection of the gel components as separate and distinct slugs into the treatment region is essential to the teaching of treatment processes such as Gall insofar as surface contacting of the polymer and crosslinking agent was believed to cause premature gelation of the gel components prior to reaching the treatment region. If prematurely formed at the surface or in the well bore, placement in the treatment region of the gels taught by Gall was difficult, if not impossible, to achieve. Consequently, sequential injection of the gel components in concept avoided premature gelation by delaying contacting of the components until they were displaced out into the treatment region. In situ contacting of the polymer and crosslinking agent as required by Gall, nevertheless, proved operationally unattractive in many hydrocarbon recovery applications because of the difficulty in achieving adequate mixing of the gel components in situ. Without adequate mixing, the gels of Gall were poorly formed, resulting in weak and unstable gels which performed ineffectively as permeability reducing agents.
In response to the shortcomings of sequential injection processes such as Gall, U.S. Pat. No. 4,683,949 to Sydansk et al identified specific gel components and gelation parameters for a crosslinked polymer gel having utility in a conformance improvement treatment process, whereby the polymer and crosslinking agent of the gel could be mixed at the surface in a homogeneous gelation solution and placed in the treatment region by injection therein as a single slug. Thus, Sydansk et al overcame the inherent operational limitations of processes such as Gall which required sequential injection and in situ mixing of the gel components. The gel technology of Sydansk et al was predicated on the finding that a chromium(III)/carboxylate complex crosslinking agent could be mixed with a crosslinkable polymer at the surface to form a gelation solution. The gel resulting from this gelation solution was uniquely stable, highly predictable and, therefore, capable of practical long-term stability and good performance in subterranean treatment regions.
The teaching of Sydansk et al has subsequently been modified to adapt the teaching to specific problematic treatment applications. For example, fractured or otherwise highly permeable formations are a difficult environment for conformance improvement treatments because the treatments require gels having a high degree of structure to effectively reduce permeability in the treatment region of such formations. U.S. Pat. No. 5,609,208 to Sydansk discloses the specific utility of gels containing a high molecular weight acrylamide polymer for treatments in high permeability formations because the degree of structure and stability of the gel containing the acrylamide polymer increases as the molecular weight of the polymer is increased. The problem is compounded, however, when the highly permeable formation being treated has a relatively high temperature. The high formation temperature causes polymer autohydrolysis which over time can contribute to destabilization of the gel utilized in the conformance improvement treatment.
Increasing the concentration of the high molecular weight polymer in the gelation solution increases the stability, strength and performance of the resulting gel, which offsets the destabilizing effect of high temperature. However, continuously increasing the concentration of the high molecular weight polymer eventually creates unacceptably high viscosities in the gelation solution which may lead to mixing and dissolution problems and excessive pressure drops in well tubulars and difficulty in preparing the gelation solution, particularly if the polymer is in a dried solid state, and in placing the gelation solution in the treatment region. As such, a need exists for a process which produces a gel having utility in permeability reduction and fluid mobility control treatments, wherein the gel has increased stability, strength and performance, yet the gelation solution from which the gel is produced does not exhibit excessively high viscosities.
Accordingly, it is an object of the present invention to provide a process which forms a crosslinked polymer gel for permeability or fluid mobility reduction in or proximate to a subterranean hydrocarbon-bearing formation, wherein the gel has increased stability, strength and performance. It is also an object of the present invention to provide a gelation solution which forms the crosslinked polymer gel, wherein the gelation solution does not exhibit excessively high viscosities. It is more particularly an object of the present invention to form a gel from a gelation solution which has specific utility for the treatment of high-temperature, highly permeable formations. It is still more particularly an object of the present invention to form a gel from a gelation solution which has specific utility for the treatment of high-temperature, fractured carbonate formations. These objects and others are achieved in accordance with the invention described hereafter.